SIP AG Registration

SIP AG Registration Process

Before a SIP user initiates a call, the user must register user information on the home network. After the user is added, the user registration process starts.

A SIP AG user needs to register with the IMS core when the user initiates a call for the first time. Figure 3-2 shows the registration process.

1. The SIP AG sends a REGISTER message to the IMS core.
2. If the IMS core requires authentication but cannot find user information in the database, it sends a 401 message, which contains information required for authentication.
3. The SIP AG asks the user to enter the user name and password. It encrypts the user name and password, encapsulates them into a REGISTER message, and sends the REGISTER message to the IMS core.
4. The IMS core decrypts the user information from the REGISTER message and sends the user information to the authentication/accounting center. After the user is authenticated, the PBX records the user information in the database and sends a 200 OK message to the SIP AG.

Figure 3-2 SIP AG registration process

SIP AG Registration Redirection Process

If a SIP AG obtains the IP address and port number of the remote end through DNS, the SIP AG can be redirected for registration. By using the DNS, the SIP AG can obtain one or more IP addresses. The address can be either an IMS core address or a redirection server address. When a user initiates a call for the first time, the SIP AG sends a REGISTER message to the IP address obtained through the DNS. If this IP address is a redirection server address, the SIP AG initiates registration redirection.

Figure 3-3 shows the SIP AG registration redirection process.

1. When a user initiates a call for the first time, the SIP AG sends a REGISTER message to the redirection server.
2. The redirection server sends a 301 Moved Permanently message to the SIP AG, which contains a redirection address. When receiving the 301 Moved Permanently message, the SIP AG verifies whether the redirection address is in the IP address list obtained through the DNS. If the address is not in the list, the SIP AG repeats Step 1 and Step 2 to obtain a new redirection address, until it obtains a redirection address that is in the IP address list.
3. If the address is in the list, the SIP AG sends a REGISTER message to the corresponding IMS core.
4. After the IMS core receives the REGISTER message, it verifies the validity of user information, saves the information in the database, and sends a 200 OK message to the SIP AG. After registration redirection, the SIP AG connects to the redirected IMS core to perform all interactions.

Figure 3-3 SIP AG registration redirection process

SIP AG Deregistration Process

1. The SIP AG sends a REGISTER message (deregistration request) to the IMS core.
2. When receiving the REGISTER message, the IMS core sends a 200 OK message to the SIP AG and deregisters the user.

Figure 3-4 SIP AG deregistration process